The number of echoes combined into a single image or image set in multiple echo imaging sequences or echo-train techniques such as rapid acquisition with relaxation enhancement (RARE), fast-spin echo (FSE), and turbo spin-echo (TSE). In RARE imaging, the ETL typically equals the acceleration factor.
As for tooth pain, it is caused by a reaction of the nerves inside a tooth's pulp chamber with the severity dependent upon the type and degree of the stimulus. What follows are some examples of symptoms you may be feeling and their possible causes.
Small cavities are occasionally sensitive to sweets, hot and cold. The sensitivity usually disappears once a filling is placed. Large cavities or a hit to the tooth and/or jaws can cause nerve damage inside the tooth. This pain is more constant and severe and may be referred to more as a throbbing pain. The most common signs of nerve damage are pain when biting, chewing, drinking something hot or cold, and swelling of the face or gums.